Inkjet inks typically include a colorant, such as a pigment or a dye, in an ink vehicle. When applied to a print medium, the colorant is absorbed into the print medium and produces a printed image. Ideally, once printed, the printed image is permanent and does not fade or degrade over time. However, in actuality, the printed image tends to fade upon exposure to gases or pollutants. This phenomenon is referred to herein as “gasfade” and is also commonly referred to as “airfade.” The printed image fades due to atmospheric pollutants, which degrade or decompose the colorants. These atmospheric pollutants include oxygen (“O2”), ozone (“O3”), sulfur dioxide (“SO2”), and nitrogen oxides (“NOx”), such as nitric oxide (“NO”), nitrogen dioxide (“NO2”), nitrogen trioxide (“NO3”), and mixtures thereof. Since many of these atmospheric pollutants are present in air, the printed image will fade even when stored under optimal conditions, such as in a museum or in another controlled environment. O3 is present in ambient air, such as inside homes, offices, or other buildings, at 40-150 parts per billion by volume (“ppbv”), depending on the location, season, weather, and time of day.
The colorant fades due to photodegradation mechanisms, which include oxidation or reduction of the colorant, electron ejection from the colorant, reaction with ground-state or excited singlet state oxygen, and electron or hydrogen atom abstraction to form radical intermediates. The atmospheric pollutants generate free radicals that degrade the inkjet ink and/or the print medium and generate more free radicals, which further accelerate the degradation process.
While gasfade is observed in images printed with either dye-based or pigment-based inkjet inks, it is more pronounced with dye-based inkjet inks. Furthermore, while gasfade is observed on different types of print media, it is especially pronounced when the image is printed on a porous print medium. Porous print media are known in the art and typically include an ink-receiving layer that is formed from porous, inorganic particles bound with a polymer binder. The inkjet ink is absorbed into the pores of the inorganic particles and the colorant is deposited on the surface of the inorganic particles. Porous print media have a short dry time and good resistance to smearing because the inkjet ink is easily absorbed into the ink-receiving layer. However, due to their porous nature, porous print media do not exhibit good resistance to gasfade. Gasfade is less pronounced on swellable print media, which have synthetic or natural polymers that swell when contacted with the inkjet ink. Swelling of the polymer encapsulates the colorant in a coating, which protects the colorant, to a certain extent, from atmospheric pollutants.
Gasfade in porous print media has only recently been identified as a significant problem and, therefore, few solutions to this problem have been proposed. One proposed solution is to add metal oxides to the print media. Alternatively, low molecular weight hindered amine light sensitizers (“HALS”), antioxidants, and UV absorbers are added to the print media. However, these additives are sacrificial and do not provide long term protection. Another proposed solution includes forming a barrier layer over the printed image using lamination techniques. While the barrier layer effectively reduces gasfade, the barrier layer is time consuming to apply and cost intensive.
Some atmospheric pollutants, such as NOx and O3, are known to react with sulfide functional groups. For instance, NOx and O3 react with sulfides as shown in the reaction scheme below:
where R is an alkyl group, an aryl group, or a polymer. The O3 or NOx oxidize the sulfide groups to sultone groups, sulfone groups, or sulfonate groups. The sulfonate group is then converted to sulfonic acid.
Sulfur-containing compounds have been used to filter or remove ozone from gases. For instance, poly(1,4-phenylene sulfide), sodium sulfite, or sodium thiosulfate have been used to remove ozone from air samples containing reactive volatile organic compounds. Non-sulfur containing compounds, such as potassium iodide, potassium carbonate, and manganese dioxide-coated copper have also been used. Poly(phenylene sulfide) (“PPS”) has also been used as a filter material to selectively remove ozone from gas samples that contain ozone and organic substances. In addition, PPS has been used to remove ozone from liquid samples. To remove the ozone, the gas or liquid samples are passed through a solid or crystalline form of the sulfur-containing compounds. Alternatively, the gas or liquid samples are passed through a substrate impregnated with the sulfur-containing compounds.
It would be advantageous to reduce gasfade on print media, such as porous print media. In addition, it would be advantageous to provide long term protection against gasfade.